cli: fix resetting values via property alias

Property aliases should really just be shortcuts for one fully spelled
out property (sometimes, they do more like "master").

Anyway, we must also handle resetting the value, otherwise:

  $ nmcli connection add type gsm apn ""

will still result in "gsm.apn=internet", unlike

  $ nmcli connection add type gsm gsm.apn ""

NEWS

@@ -10,7 +10,9 @@ USE AT YOUR OWN RISK.  NOT RECOMMENDED FOR PRODUCTION USE!
 
 * Add an 'accept-all-mac-addresses' property to the ethernet setting
   to accept frames with any MAC address (also known as promiscuous
-  mode)
+  mode).
+* nmcli: fix setting property aliases to empty value to reset the
+  default value.
 
 =============================================
 NetworkManager-1.30

src/nmcli/connections.c

@@ -4182,23 +4182,17 @@ set_option(NmCli *                   nmc,
                        NULL);
     if (option && option->check_and_set) {
         return option->check_and_set(nmc, connection, option, value, error);
-    } else if (value) {
-        return set_property(nmc->client,
-                            connection,
-                            setting_name,
-                            property_name,
-                            value,
-                            inf_flags & NM_META_PROPERTY_INF_FLAG_MULTI
-                                ? NM_META_ACCESSOR_MODIFIER_ADD
-                                : NM_META_ACCESSOR_MODIFIER_SET,
-                            error);
-    } else if (inf_flags & NM_META_PROPERTY_INF_FLAG_REQD) {
-        g_set_error(error,
-                    NMCLI_ERROR,
-                    NMC_RESULT_ERROR_USER_INPUT,
-                    _("Error: '%s' is mandatory."),
-                    option_name);
-        return FALSE;
+    } else {
+        set_property(nmc->client,
+                     connection,
+                     setting_name,
+                     property_name,
+                     value,
+                     !value ? NM_META_ACCESSOR_MODIFIER_DEL
+                            : (inf_flags & NM_META_PROPERTY_INF_FLAG_MULTI
+                                   ? NM_META_ACCESSOR_MODIFIER_ADD
+                                   : NM_META_ACCESSOR_MODIFIER_SET),
+                     error);
     }
 
     return TRUE;